1. Field of the Invention
This invention relates to a transponder which sends back a signal automatically upon reception of a signal from the outside, and in detail, to a radar transponder having characteristics that its lowest effective receiver sensitivity is below -50 dB and its effective isotropic radiation power is over +26 dBm.
2. Description of the Prior Art
In general, there has been used a radar transponder which has been installed, for example, at a port of a harbor, on a sea where many rocks are present, or at the top of a mountain difficult to approach to send out an answer signal automatically in response to a signal sent from a remote control office.
FIG. 1 is a block diagram showing a conventional search and rescue radar transponder. In FIG. 1, numeral 1 is a receiving antenna, numeral 2 is a Field Effect Transistor (FET) amplifier connected to the receiving antenna 1, numeral 3 is a diode direct detector connected to the FET amplifier 2, numeral 4 is a video amplifier connected to the diode direct detector 3, numeral 5 is an auxiliary video amplifier connected to the video amplifier 4, numeral 6 is a control circuit connected to the auxiliary video amplifier 5, numeral 7 is a transmitting gate circuit connected to the control circuit 6, numeral 8 is a sweep signal generator connected to the transmitting gate circuit 7, numeral 9 is a microwave oscillator connected to the sweep signal generator 8 and the transmitting gate circuit 7, numeral 10 is a transmitting antenna connected to the microwave oscillator 9, and numeral 11 is a receiving on-off switch which is connected to the transmitting gate circuit 7 and outputs a signal to the FET amplifier 2.
Next, the operation will be described. A signal which is sent from a search radar and received by the receiving antenna is amplified in the FET amplifier 2 and sent to the diode direct detector 3. The detected output from the diode direct detector 3 is amplified to 60 dB or so by the video amplifier 4 and further, amplified by the auxiliary video amplifier 5 up to a level necessary for triggering the control circuit 6. When the control circuit 6 is triggered by the output of the auxiliary video amplifier 5, it generates a pulse for producing a transmitting time of the radar transponder and supplies the pulse to the transmitting gate circuit 7.
The transmitting gate circuit 7 produces a transmitting gate pulse based on the pulse supplied from the control circuit 6 and sends out the transmitting gate pulse to the sweep signal generator 8, the microwave oscillator 9, and the receiving on-off switch 11. The sweep signal generator 8 produces a required number of saw-tooth wave signals using the transmitting gate pulse from the transmitting gate circuit 7 and supplies them to the microwave oscillator 9. The microwave oscillator 9 makes a microwave oscillation in a certain time interval by means of the transmitting gate pulse from the transmitting gate circuit 7. The oscillation frequency is varied in accordance with the voltage value of the saw-tooth wave signal from the sweep signal generator 8 within the rated frequency range for frequency sweeping. The transmitting output which is supplied from the microwave oscillator 9 and frequency-modulated therein is radiated through the transmitting antenna 10 to space.
When the receiving on-off switch 11 is applied with the transmitting gate pulse from the transmitting gate circuit 7, it stops the operation of the FET amplifier 2 only during the period corresponding to the width of the receiving gate pulse by making the output level supplied to the FET amplifier 2 zero. This attenuates the input applied to the diode direct detector 3 and further attenuates the inputs applied to the video amplifier 4 and the auxiliary video amplifier 5. Thus, the radar transponder is made so as not to suffer influences due to its own transmitting power.
A conventional search and rescue radar transponder is constituted as described above. Accordingly, only during the transmitting period, the operation of the FET amplifier 2 is stopped and the input to the video amplifier 4 is attenuated. But, since the input can not be broken completely, although a transmitting output fed back to the receiving antenna 1 during the transmitting period is attenuated by stop of the operation of the FET amplifier 2, the transmitting output is amplified to 60 dB or so by the video amplifier 4, and an input signal having a substantially fairly high level is input to the auxiliary video amplifier 5. There has been a problem that owing to the influence of the above process, the video amplifier 5 can not operate normally for a long time, and the transponder can not receive signals from search radars even after the transmission is stopped.